This invention relates to digital color copying systems, and specifically to a copying system wherein the raw RGB signals from the scanner is transformed into a calibrated colorimetric space so that the color signals may be further manipulated.
One form of modem color document scanner includes an illuminant light source, three color filters (R. G, and B), and a CCD detector. The illuminant is a wide band artificial light source that is different from the viewing illuminant. The light reflected from the target is filtered by the RGB filters before reaching the CCD detector, thus producing a three-channel RGB signal. If the combined spectral responses of the illuminant, the RGB filters, and the CCD detector are a linear combination of the CIE Color Matching Functions (CMFs), a color scanner may see colors exactly as seen by the human visual system (HVS), through proper processing. However, this situation generally does not occur because of necessary compromises of light source efficiency and the cost associated with designing and manufacturing color filters. It is possible that a color scanner sees two distinct colors as the same color, while the human visual system views the colors as different, or vice versa. This is the scanner metamerism problem. It makes the color characterization of a scanner extremely media and colorant dependent. A scanner characterization that works well for one set of media and colorant combination will usually make use of the special spectral characteristics of that combination and therefore works poorly for other media and colorant combinations.
In order to obtain meaningful colorimetric values from a RGB scanner, color characterization must be performed on the scanner. The characterization procedure begins by scanning a set of color patches for a certain colorant and media combination to obtain raw scanner RGB values. The color patches are also measured by a spectrophotometer, such as a Gretag Spectrolino, to obtain their CIE XYZ or Lab values. After these two sets of data are obtained, regression and/or interpolation techniques may be used to establish a mathematical model for converting scanner RGB values to CIE relatable values, such as CIE XYZ or Lab values, or other output matrices.
The two commonly used mathematical models are based on matrices and 3D look-up-tables (LUTs). The recently published ICC Profile Format Specification, International Color Consortium, ICC profile format specification, Version 3.4, Aug. 15, 1997, contains scanner profile formats based on these two models. In general, these techniques produce satisfactory results for a specific colorant and media combination. However, because of scanner metamerism, a scanner transform generated for one colorant and media combination will not work well for another combination. Many scanning applications in the graphic arts acquire images from a limited number of colorant and media combinations, such as reflection photographic prints, color transparencies, or color negative films. A specific color transform is made available for each colorant and media combination, resulting in quite accurate color conversion. Scanner metamerism is not an issue in such use.
The documents to be scanned by a digital color copier, however, originate from a variety of sources and include many different colorant and media combinations. These combinations include, among others, photographic prints, graphic arts materials, ink on various ink jet papers, and laser printer outputs. It is not practical to require a typical user to identify the colorant and media to be scanned. Therefore, only one transform may be provided for color conversion across different colorant and media combinations. One commonly used approach is to average individually generated transforms to produce a combined color conversion transform. The problem with this approach is that although an individual transform works well for its intended colorant and media combination, the combined color conversion transform may result in error prone color conversion across different media.
Attempts have been made to produce colorimetric digital input devices, as described by P. G. Engeldrum, Color scanner colorimetric design requirements, xe2x80x9cSPIE Vol. 1909, 1993, and R. E. Burger, Device independent color scanning, SPIE Vol. 1909, 1993, however, such devices have not been successful. A scanner built according to Engledrum or Burger is quite expensive, as the combined spectral responses of digital filters, light source, and CCD detector must correspond closely to CMFs. Most modern scanners are, at best, approximately colorimetric and accurate color conversions for them are still colorant and media specific.
The following patents describe methods for calibrating a RGB scanner to make it produce color values in a colorimetric space.
U.S. Pat. No. 4,060,839 to Sakamoto, for Method of color correction, granted Nov. 29, 1977, provides correction to a RGB signal in CMYK color space.
U.S. Pat. No. 4,342,047, to Niemczyk et al., for Color calibration accuracy in a color raster scanner, granted Jul. 27, 1982, describes a scanner which analyzes small area of an image containing a single color only, and moving a scan head about in that area.
U.S. Pat. No. 5,149,960 to Dunne et al., for Method of converting scanner signals into colorimetric signals, granted Sep. 22, 1992, describes sensing an input image to generate an input signal and processing the input signal in an iterative correction loop.
U.S. Pat. No. 5,200,817 to Birnbaum for Conversion of an RGB color scanner into a colorimetric scanner, granted Apr. 6, 1993, describes an RGB input which is transformed to an equivalent neutral density (END)-like RGB signal, which is then transformed to a tristimulus XYZ signal.
U.S. Pat. No. 5,452,112 to Wan et al., for Color image reproduction system field calibration method and apparatus, granted Sep. 19, 1995, uses a scanning target, reference files and an image to generate a printer calibration table.
U.S. Pat. No. 5,491,568 to Wan, for Method and apparatus for calibrating a digital color reproduction apparatus, granted Feb. 13, 1996, uses tri-linear interpolation based on a 3D LUT to implement a color transform.
U.S. Pat. No. 5,543,940 to Sherman, for Method and apparatus for converting color scanner signals into colorimetric values, granted Aug. 6, 1996, requires designation of a specific medium to construct a calibration model for a scanner.
The following patents describe scanner color calibration.
U.S. Pat. No. 4,346,402, to Pugsly, for Image-reproduction apparatus, granted Aug. 24, 1982, uses a 3D LUT or ROM to provide color-corrected signals.
U.S. Pat. No. 4,929,978 to Kanamori et al., for Color correction method for color copier utilizing correction table derived from printed color samples, granted May 29, 1990, uses sample colors that are compared to scanned values to determine proper output.
U.S. Pat. No. 5,185,661 to Ng, for Input scanner color mapping and input/output color gamut transformation, granted Feb. 9, 1993, describes scanner color calibration using a 3xc3x973 transform matrix and a color gamut compression list.
The following references describe methods which are embedded into RGB scanners to make the scanner output signals more colorimetric:
U.S. Pat. No. 5,285,271 to Gennetten, for Digital color matrix circuit, granted Feb. 8, 1994, transforms the intensity of each encoded color.
U.S. Pat. No. 5,665,963 to Campbell, for Reflective color filter for color correction of photodetector filters, granted Sep. 9, 1997, uses a wave-length specific filter to suppress IR and longer-wave visible red light.
U.S. Pat. No. 5,773,814 to Phillips, et al., for Sensor assembly providing gray scale and color for an optical image scanner, granted Jun. 30, 1998, describes a sensor array having three sub-arrays, including one white (unfiltered) sub-array and two color sub-arrays, wherein only the white sub-array is used for grey-scale sensing and wherein all three sub-arrays are used to sense color.
The following references describe various scanner operations and features: K. D. Gennetten and M. J. Steinle, Designing a scanner with color vision, Hewlett-Packard Journal, August 1993; Lee et al., Linear model of surface and scanner characterization method, SPIE Vol. 2414, 1995; Hung, Colorimetric calibration for scanners and media, SPIE Vol. 1448, 1991; Holub et aL, Color systems calibration for graphic arts: 1. Input devices, Journal of Imaging Technology, Vol. 14, No. 2, April 1988; and International Color Consortium, ICC profile format specification, Version 3.4, Aug. 15, 1997.
A method of transforming scanned RGB values into a colorimetric color space includes identifying multiple input media types to be scanned; assigning a weighting factor to each input media type; dividing input color space into segments; assigning a weighting factor to each input color space segment; applying regression and interpolation techniques to minimize color error; and generating a colorimetric output.
An object of the invention is to provide a method for using weighting factors to obtain optimal scanner conversion transforms across different colorant and media combinations.
Another object of the invention is to manipulate an RGB input to obtain a CIE colorimetric color output.
A further object of the invention is to provide weighting factors to optimize HVS perception of an output image.